1. Field of the Invention
The invention generally relates to a method of mixing crystalline solids with polymeric materials and, more particularly, to a methodology which utilizes supercritical fluids, such as supercritical carbon dioxide, to combine solid materials with polymeric materials in a homogeneous blend.
2. Background of the Invention
Polymer technology has employed supercritical carbon dioxide as an alternative fluid medium to replace harmful organic solvents. The use of supercritical carbon dioxide in the synthesis of polymers is discussed in Cooper, A. I., J. Mater. Chem. 10:207 (2000); Ajzenberg et al., Chem. Eng. Technol. 23(10), 829 (2000), and U.S. Pat. Nos. 5,496,901 and 5,618,894, which are herein incorporated by reference. These references describe solubilizing monomers, including fluoromonomers, in carbon dioxide, and then polymerizing the solubilized monomers to form polymers of interest. Supercritical carbon dioxide has also been used to extract low molecular weight components from polymer matrices as discussed in McHugh et al. Supercritical Fluid Extraction, Butterworth-Heineman, 1994. In addition, supercritical carbon dioxide has been used as a blowing agent for the production of polymer foams as discussed in Utracki et al. J. Polym. Sci. Part B-Polymer Physics 39(3), 342 (2001) and Cooper, ibid. Furthermore, supercritical carbon dioxide is used for polymer processing. See, Kwag et al., Ind. Eng. Chem. Res. 40(14), 3048 (2001) and Royer et al., J. Polym. Sci. Polym. Physics 38(23), 3168 (2000). Coating applications require dissolution or suspension of polymer in solvent. Supercritical carbon dioxide has been used as a solubilizing and suspending media because of its benign nature and solvent characteristics as a function of temperature and pressure in the supercritical state. See, for example, U.S. Pat. Nos. 5,696,195, 6,034,170, and 6,248,823, all of which are herein incorporated by reference. There is a high affinity of amorphous fluoropolymers for supercritical carbon dioxide (see, Kazarian, J. Amer. Chem. Soc. 118(7), 1729 (1996). This may be due to interactions between carbon dioxide molecules in the supercritical phase and C═O and C—F bonds in the fluoropolymer.
Several other references are related to combinations of fluoropolymers and supercritical fluids, such as carbon dioxide. U.S. Pat. No. 5,530,049 to Tuminello describes compositions of perflourinated polytetrafluoroethylene (PTFE) dissolved in supercritical carbon dioxide. Japanese Patent Application 98233244 describes purification of fluoropolymers by dissolving the fluoropolymer in a medium which contains a supercritical fluid. U.S. Pat. No. 6,034,170 to Dee describes compositions of supercritical carbon dioxide and fluoropolymers where the ratio of hydrogen atoms to fluorine atoms is controlled. U.S. Pat. No. 5,821,273 describes the use of supercritical carbon dioxide as a foaming agent for fluoropolymers. U.S. Pat. No. 5,863,612 to Desimone describes preparing fluoropolymers from a composition including fluoromonomers in supercritical carbon dioxide. U.S. Pat. No. 5,645,894 to Trankiem discloses coating a razor blade with PTFE using a dispersion of PTFE in supercritical fluid. Japanese Patent Application JP 91205307 describes a fractionating process involving fluorochemicals and supercritical or subcritical carbon dioxide. U.S. Pat. No. 5,696,195 to McHugh describes the production of foams or webs using supercritical sulfur hexafluoride and tetrafluoroethylene polymer.
Fluoropolymers have superior chemical and solvent resistance characteristics, and excellent thermal stability. Because of these properties, fluoropolymers have been increasingly used in the chemical and semiconductor industry. However, processing of certain fluoropolymers, particularly PTFE and “modified” PTFE, can be difficult. For example, PTFE is not moldable or extrudable. Rather, PTFE components are typically cut or shaved from billets of material. PTFE is often processed by techniques that resemble those for ceramics. PTFE is sintered at 370° C. before being formed by processes such as ram extrusion (see Scheirs, Modern Fluoropolymers, John Wiley & Sons, New York, 1997). Recently, fluoropolymer thermolysis (burning) and sintering has been identified as a potential source of halogenated organic acids in the environment (see Ellis et al., Nature 412:321 –324 (2001)). These halo-acids, such as trifluoroacetic acid are persistent in the environment, as they have no known degradation process (see Boutonnet et al., Human and Ecological Risk Assessment 5:59 –124 (1999)).
U.S. Pat. No. 5,567,769 to Noda describes the preparation of a homogenous blend of a styrene and methyl methacrylate or 1,2 butadiene which involves the uses of supercritical fluid. In the process, the polymers, which are thermodynamically immiscible, are expanded through a narrow opening, and then fluid evaporation results in the deposition of a homogenous material. A similar technology is described in U.S. Pat. No. 5,290,827 to Shine.
U.S. Pat. No. 5,548,004 to Green describes the production of a coating powder made from two different organic materials which are combined with supercritical carbon dioxide, agitated in situ, and then discharged into a vessel that is at lower pressure than the vessel in which the constituents are combined. U.S. Pat. No. 5,766,522 to Daly describes the production of a powder coating wherein PTFE and/or a thermosettable resin developed in a continuous extruder using supercritical carbon dioxide and a stream of precursor chemical. The process produces a powder coating. Similar technology is described in U.S. Pat. No. 6,114,414 to Daly. U.S. Pat. Nos. 5,672,667 and 5,530,077 to Desimone describe multiphase mixtures which are prepared with a variety of polymers in supercritical carbon dioxide.
German Patent De 42023320 to Benken describes using a supercritical fluid such as carbon dioxide or an alkane to carry an impregnating material into the pores of a substrate. For example, flavorants or aromas can be impregnated into food and cigarette products. U.S. Pat. No. 5,968,654 to Lee describes modifying a polymer substrate by contacting it with a fluorinated compound distributed within a supercritical fluid.